High Q smd capacitors

Right. It has to drive headphones.

Wouldn't you want the power from both sidebands?

The ideal crystal radio would use a matched-impedance-both-ends bandpass filter.

I don't know if it is not understood, when I say the Q of my coil and cap or LC if XXXX that is means just the coil and cap or LC, Yes, that is Qu now doubt about it. Qu, Qu! .

You will need to give me more info, maybe one of your hand drawn schematics.

I ask on a crystal group, "Have you built a radio with Q so high it limits Bandwidth?"

So far two responses,

"It's hard to imagine a practical LC combination with that much Q. And tunable, too. The idea of clipping the audio of much of what's on the AMBC band these days does sound appealing, though."

And,

"I have been hearing that "anti high-Q" / "clipping" crap for years on forums. Mostly from so called "experts" and angry old hams who want to dispute the advantages of litz wire in crystal radios (having never used it). If it was the case, then nobody would listen to regen rceivers, specially at MW. (The same people are happy to exhort the virtues of regens.)"

Mikek

Here's a video showing audio reducing as the frequency increases. He says Q=1190, but I don't know if that was loaded Q. I suspect yes, but then how did he do that? It may have been a different test, it did come from a crystal radio forum

Mikek

I've never heard of liquid filled capacitors. When it comes to variable ca pacitors the two choices for high Q are air and vacuum. I thought you coul d add a solid dielectric to increase the voltage and capacitance of an air variable cap, but the air gap, no matter how small it is, ends up with the lion's share of the voltage which would result in corona discharge at the s urface of the dielectric. Not good.

I found some good liquid dielectrics, but the mechanics of a liquid variabl e cap are too complex to make practical.

But then you likely aren't talking about variable caps. But this is for a resonant circuit, right? Are you planning to adjust the coil?

The cap is being used to resonate the coil. Then the tool counts how many cycles it takes to ring down to a certain voltage. The author used a fixed capacitor to do this. This was to avoid the cost of a Variable air cap. What I wanted to know was, is the Q of the cap high enough, compared to an air cap, so as to have little effect on measured Q. I didn't have a Q of the capacitor, I do have numbers on how good or how bad an air cap can be. Mikek

e capacitors the two choices for high Q are air and vacuum. I thought you could add a solid dielectric to increase the voltage and capacitance of an air variable cap, but the air gap, no matter how small it is, ends up with the lion's share of the voltage which would result in corona discharge at t he surface of the dielectric. Not good.

iable cap are too complex to make practical.

r a resonant circuit, right? Are you planning to adjust the coil?

Ok, so you don't need a variable cap. That doesn't mean you can't use an a ir cap with a very, very high Q so you don't even need to consider it. Wha t value cap do you need? An air cap is just a couple of plates separated b y air. Their value can be calculated easily even with the fringe effects.

I would say the Q of the cap needs to be on the order of 10x the coil to me asure it without a significant influence. Of course if you know the cap Q and it is stable you can factor that in, but unless it is very well known a nd stable it still needs to be higher than the coil to get a good measureme nt.

If I was build his circuit I would put in a air variable, I have 6 good high Q caps and a many other plain old air variable caps. The author chose to use an smd cap to reduce cost. My inquire regarded whether he degraded the machine by using that smd cap. The cap I was questioning was the smd on the sales sheet listed in this thread.

So the question boils down to this, What is the Q of the cap in that sales sheet? Probably can't find that info, so, what is the Q of a high Q cap in values between 21pf and 448pf smd capacitor?

Reading the 9th post in that thread the author says the smd caps have higher Q than the air variable. I'm skeptical but now curious.

Mikek

It is a different thing from the electronics or radio technology we are used to - in a way similar to audiophiles...

ood

ble capacitors the two choices for high Q are air and vacuum. I thought yo u could add a solid dielectric to increase the voltage and capacitance of a n air variable cap, but the air gap, no matter how small it is, ends up wit h the lion's share of the voltage which would result in corona discharge at the surface of the dielectric. Not good.

ariable cap are too complex to make practical.

for a resonant circuit, right? Are you planning to adjust the coil?

an air cap with a very, very high Q so you don't even need to consider it. What value cap do you need? An air cap is just a couple of plates separat ed by air. Their value can be calculated easily even with the fringe effec ts.

o measure it without a significant influence. Of course if you know the ca p Q and it is stable you can factor that in, but unless it is very well kno wn and stable it still needs to be higher than the coil to get a good measu rement.

Until you know exactly what caps the guy used you can't say if his measurem ents were degraded.

BTW, it was the 7th post. I finally found you in the 8th post.

You can't know what that guy's cap was, so why not focus on your own caps? An air cap (btw, I don't know what they mean by VC, variable cap or vacuum cap?) almost certainly will have better Q than any solid dielectric cap. Q is reduced by energy losses. Air caps have exceedingly small energy los ses. Most likely they are dominated by the AC resistance of the conductors . The dielectric material can dissipate energy from the AC voltage realign ing the polar moments within the dielectric.

Anything is possible, but it is very hard for me to imagine an air cap will have worse Q than virtually any solid dielectric cap. However a good non- air cap might be pretty close. Alumina and Silica can have very low dissip ation factors. I'm just not sure that conveys to the material used in caps . I did find mica caps which provide very low loss over a wide frequency r ange.

Digikey seems to have some for a couple/three bucks each.

The Q of a cap may not be given in a data sheet since that is not the way t hey are typically listed. Instead look for the other ways of describing th e same thing.

"a capacitor's loss tangent is sometimes stated as its dissipation factor, or the reciprocal of its quality factor Q, as follows

tan ?? = DF = 1/Q "

Sometimes they list the dissipation factor other times I find dissipation f actor angle. One is the angle, the other is the tangent of the angle, smal ler is better obviously. In radians, the two values are pretty much equal for small values which is what you are looking for. For small enough angle s, the angle, the sin and the tan are all equal.

Not at all! If you build a piece of equipment to measure the Q of an inductor and then use a capacitor that has the same Q as your inductor your tool is automatically going measure 50% low. So the Q of the resonating cap is important. Now where is my 2 gauge oxygen free litz speaker wire and my pyramid shaped floor supports for the wire? Mikek

lossy capacitor

The part I don't get is the contribution (or detraction) to Q from the ulti mate load of the resonator circuit. In this circuit, how do you measure th e ring down without sucking energy out and impacting your measurement? So there are three factors in the equation.

The in the application circuit a high Q gives a high sensitivity and select ivity. But again, how do you pull signal from the antenna into the receive r without lowering the Q?

It's a similar problem to measuring the Q of the coil with a low Q capacito r. If the power draw of the receiver input is not significantly lower than the power in the antenna or resonator circuit the Q will be impacted. At some point a higher Q will accomplish nothing measurable.

Only if there's no self-calibration.

Cheers

Phil Hobbs

I'm sure it does, usually you see a high input impedance FET front end to limit loading. This circuit has a series 3pf cap to a FET gate, the bias, basically has two 5.6 Meg resistors to ground as the load. Not to bad, the gurus here would have some bootstrapped circuit that was

You don't, to get audio you must extract some power to get the audio, thus it lowers Q. Since you have no amplification, you want to limit all losses as much as possible, so the starting point is a high Q inductor ans a high Q capacitor.

That's why I ask, but it seems the caps have are pretty high Q.

Mikek

Guru! :-)

Mikek